This invention relates generally to alternating current power sources and more particularly, but not by way of limitation, to a reserve alternating current power supply for a computer.
To power electrical devices there is the need for some source of power. Quite frequently, the power is to be of the alternating current (AC) type rather than the direct current (DC) type. Although the electrical device might ultimately require DC power, such DC power can be obtained from an AC power source, such as by a rectification process as known to the art. Therefore, there is the general need for an alternating current power source.
By way of a specific example, an electronic computer needs to be energized by some type of power source. Although components of the computer might ultimately require DC power, AC power is frequently used as the primary source from which the DC power can be derived. Therefore, in energizing an electronic computer, the commercial AC power supply obtained from a wall socket or the like is frequently used. However, because the commercial AC power supply is susceptible to power interruptions, such as outages or brown-outs, there is the further need for a second alternating current power source which can function as a back-up to the commercial supply or, alternatively, as the primary supply if so desired. That this need has arisen is indicated in U.S. Pat. No. 4,038,559 to Chun et al. which discloses a system having two alternating current power sources and which also discloses other prior art techniques.
Regardless of the specific use of such an alternating current power source, there is the need that such a power source properly control its power output by monitoring the power output.
When functioning in a reserve mode, such a power source should automatically detect power interruptions and apply its power to prevent damage to or improper operation of the electrical load being powered.
Such a power source functioning in a reserve mode should also automatically monitor and reconnect the primary power source to the load when the primary power source returns to proper operation. Disconnection of the reserve power source should also be performed by the power source once the primary source has been reconnected.
To insure that the reserve power source will be available when a primary power interruption occurs, the reserve power source should have the capability of being recharged by the primary power source when the primary power source is operating properly.
Indications perceptible to an operator should also be provided by such a power source so that the present operating status can be quickly determined.
When such a power source is functioning in a reserve mode, it is also desirable that, at the time of switchover from the primary power supply to the reserve power source, the phase relationship between the two power signals provided by the primary power supply and the reserve power source be properly maintained. If an improper phase relationship is not properly adjusted, a transformer used in the reserve power source or a transformer on the input of the load can saturate due to the resultant flux imbalance. Saturation causes the reserve power source to improperly operate.
This desirable goal of maintaining a proper phase relationship has been recognized in the prior art. For example, large inductors have been used in transformer circuits to slow current rise times in an attempt to preclude saturation current from being applied to the transformers. This proposal has shown a significant failure rate. Additionally, excessively rated output devices or large transformers have been used; however, this is relatively expensive. Another expensive proposal has been to use elaborate circuits to monitor the primary power signal from the primary supply so that the reserve power source can be switched in at the proper phase relationship. A less expensive and yet a functional means for accomplishing this end is disclosed in the parent application. U.S. patent application Ser. No. 564,495, wherein impending transformer saturation due to an improper phase relationship is detected by the pulse width modulator circuit. This is used to terminate the output of the reserve power source until the next normal output pulse is generated. Although the resultant delay between such output pulses is not detrimental in many cases, in some cases it can cause undesirable operation of the load being powered by the reserve power source. For example, an interruption of the flow of current to a computer may cause data to be lost.
Therefore, there is the need for a relatively simple, relatively inexpensive reserve power source wherein phase imbalances occurring at switchover from a primary power supply, which imbalances tend to create a saturation or flux imbalance condition within a drive transformer, can be quickly adjusted to prevent interruptions in the flow of current from the reserve power source to the load.